In the U.S Pat. No. 4,165,299, Aug. 21, 1979, there is disclosed a process for the preparation of a single phase vanadium(IV)bis(metaphosphate) catalyst for preparing maleic anhydride by the oxidation of unsaturated aliphatic hydrocarbons, which catalyst has an intrinsic surface area of from about 1.5 to 5.0 m.sup.2 /g., by forming a solid state mixture of vanadyl sulfate and phosphorus pentoxide and heating the mixture to a temperature of at least about 325.degree. C. to liberate gases and form a vanadium phosphorus reaction product which product is cooled, washed with water, dried and calcined in air to form the catalyst.
A vanadium(IV)bis(metaphosphate) catalyst which has an intrinsic surface area of only 0.30 to 0.50 m.sup.2 /g. and the preparation thereof is also shown in an article by Bruce C. Tofield et al, J.C.S., Dalton Transactions, Part II, 1975, pp. 1806-1810 and by G. Ladwig, Z. Chem., 1968, Vol. 8, p. 307, which after activation or conditioning is a good oxidation catalyst for unsaturated aliphatic hydrocarbons to produce maleic anhydride but suffers from a disadvantage of having a low intrinsic surface area. Such limited intrinsic surface area is generally undesirable because the activity of the oxides of vanadium and phosphorus are directly related to the intrinsic surface area, particularly when employed as oxidation catalysts to prepare maleic anhydride.
The typical prior art phosphorus-vanadium-oxygen catalyst systems are generally prepared by reducing vanadium pentoxide to vanadium(IV) in water or an organic solvent with hydrochloric acid or other suitable reducing agent. A source of phosphorus, usually phosphoric acid, is mixed with the vanadium (IV) solution to produce a catalyst precursor which is heat treated to give the production catalyst.
U.S. Pat. No. 4,062,873 and the numerous patent references noted therein all of which are deemed to be reiterated herein, describe various vanadium-phosphorus oxide catalyst systems employed in producing maleic anhydride.
U.S. Pat. No. 3,864,280 describes a crystalline phosphorus-vanadium mixed oxide hydrocarbon oxidation catalyst composition consisting primarily of pentavalent phosphorus, vanadium and oxygen.